Biochemical and spectroscopic studies of the electronic structure and reactivity of a methyl-ni species formed on methyl-coenzyme M reductase

The enzyme methyl coenzyme M reductase (MCR) catalyzes the final step of methane production by methanogenic organisms. The active site contains a Ni-macrocyclic complex, F-430, in which the Ni is in the 1+ oxidation state in the active form, MCRred1. We describe the preparation and spectroscopic characterization of a Ni-methyl species, denoted MCRMe, generated from MCRred1, by reaction with CH3I. EPR and C-13,1,2H pulsed ENDOR spectra of methyl isotopologues (CH3, CD3, (CH3)-C-13) umambiguously establish the presence of CH3-Ni(III)(M) moiety. They explain why both MCRred1, and MCRMe have d(x), (2)-y 2 odd-electrons although formally having Ni(I) in the former and Ni(M) in the latter. The simple MO description further gives a simple explanation to the small transfer of spin density (similar to 1%) from Ni to methyl. The MCRMe species undergoes conversion to methane and to methyl-SCoM, indicating its catalytic competence as an intermediate in methanogenesis.